In the conventional art, a speed ratio of an automatic transmission is varied hydraulically. For example, in a geared automatic transmission, a clutch is used to transmit a torque to a gear train to achieve a desired speed ratio. The clutch is provided with a hydraulic chamber to establish a pressure to engage the clutch, and a clutch piston thereof is actuated by delivering or draining hydraulic fluid to/from the hydraulic chamber to vary the hydraulic pressure therein. Meanwhile, a continuously variable automatic transmission comprises an input rotary member, an output rotary member, and a power transmission member applied to those rotary members. Both of the rotary members or any one of the rotary members is/are provided with a hydraulic chamber to establish a clamping pressure or to vary the speed ratio. Specifically, the clamping pressure for clamping the power transmission member and the speed ratio of the continuously variable transmission are varied by delivering or draining hydraulic fluid to/from the hydraulic chamber.
An example of the continuously variable transmission is disclosed in the European Patent No. EP0985855. Specifically, European Patent No. EP0985855 relates to a belt-driven continuously variable transmission comprising a pulley functioning as an input rotary member, another pulley functioning as an output rotary member, and a belt functioning as the power transmission member. Each pulley comprises a movable sheave and a fixed sheave, and each movable sheave is provided with a hydraulic chamber. According to the teachings of European Patent No. EP0985855, the speed ratio is varied by varying a running radius of the belt, that is, by moving the movable sheave by varying an amount of oil in the hydraulic chamber. Meanwhile, a belt clamping pressure of the pulley is varied by changing the hydraulic pressure applied to the hydraulic chamber. In addition, in the transmission taught by European Patent No. EP0985855, each hydraulic chamber is provided with a pressure boosting means and a pressure reduction means.
According to the transmission taught by European Patent No. EP0985855, a hydraulic system from a hydraulic source to the hydraulic chamber including valves arranged therein is structurally sealed to prevent a leakage of the hydraulic fluid. In case of raising the pressure in the hydraulic chamber, the oil is delivered to the hydraulic chamber from the pressure boosting means according to a target pressure. In addition, the pressure in the hydraulic chamber can be maintained at a constant pressure without actuating the pressure boosting means. However, if air becomes entrained with the oil in the hydraulic system in which the valves and oil passages are thus sealed, the air is also compressed when pressurizing the oil. In this situation, therefore, it is difficult to achieve a desired pressure. Moreover, a response of the automatic transmission may be slowed to degrade controllability.